Unruh deWitt detectors are important constructs in studying the dynamics of
quantum fields in any geometric background. Curvature also plays an important
role in setting up the correlations of a quantum field in a given spacetime.
For instance, massless fields are known to have large correlations in de Sitter
space as well as in certain class of Friedmann-Robertson-Walker (FRW)
universes. However, some of the correlations are secular in nature while some
are dynamic and spacetime dependent. An Unruh deWitt detector responds to such
divergences differently in different spacetimes. In this work, we study the
response rate of Unruh deWitt detectors which interact with quantum fields in
FRW spacetimes. We consider both conventionally as well as derivatively coupled
Unruh deWitt detectors. Particularly, we consider their interaction with
massless scalar fields in FRW spacetimes and nearly massless scalar fields in
de Sitter spacetime. We discuss how the term which gives rise to the infrared
divergence in the massless limit in de Sitter spacetime manifests itself at the
level of the response rate of these Unruh deWitt detectors in a wide class of
Friedmann spacetimes. To carry out this study, we use an equivalence between
massless scalar fields in FRW spacetimes with massive scalar fields in de
Sitter spacetime. Further, we show that while the derivative coupling regulates
the divergence appearing in de Sitter spacetime, it does not completely remove
them in matter dominated universe. This gives rise to large transitions in the
detector which can be used as a probe of setting up of large correlations in
late time era of the universe as well. We show that the coupling of hydrogen
atoms with gravitational waves takes a form that is similar to derivatively
coupled UdW detectors and hence has significant observational implications as a
probe of late time revival of quantum correlators.
